In the art of wood drying plants dryers comprising internal conveyors composed of a pair of belts arranged to run overlying with the wood to be dried between them, generally sheets of veneer, are known.
These types of dryers, thanks to the running of the wood sheets between the belts guided along an appropriate path, have the important characteristic of accompanying the drying action by a compression action on the surface of the wood sheets so as to counteract the phenomenon of undulation of said sheets caused by the tensions induced by heating and loss of internal humidity.
In known dryers the belts are moved by powering a part or all of the guide rollers on which the belts are wound along their path with the intention of distributing as much as possible the entraining effort of said belts. Examples of dryers made in this manner are shown, for example, in German patent no. 1,266,233, in U.S. Pat. No. 4,862,600 and European patent application no. 0,152,576, which corresponds to U.S. Pat. No. 4,654,981. Although the entraining effort along the belt is divided, it has been noted that with this type of arrangement the tension on each belt is highly variable along the path with alternation of very taut sections with almost loose sections.
In particular, since the primary factor of resistance is provided by the friction between the belts, they would tend to progressively loosen the tension, but closing them in a loop causes the loosening in a section to be reflected as tightening in another. Thus the driven rollers for entraining behave some as motor elements and some as braking elements, thus counteracting each other and causing in an uncontrollable and random manner zones of strong over-tightening and zones of accented loosening. In the former zones there are produced excessive stresses in the conveyor lines with problems of breakdown and faulty operation while in the latter zones there is a high probability of overlapping of the sheets conveyed with the imaginable treatment defects.
The general purpose of the present invention is to obviate the above shortcomings by proposing a drying plant with overlying conveyor belts with an arrangement of powered traction rollers which prevent excessive and uncontrolled stresses on the belts.
In view of said purpose it has been sought to provide in accordance with the present invention a drying plant for wood panels and in particular veneer of the type comprising two overlying conveyor belts substantially in contact along a sinuous path between guide rollers inside a drying chamber for conveyance between them of the wood panels from an input end to an output end of said chamber and characterized in that outside the path in mutual contact the two belts wind around powered running rollers, the rollers inside the sinuous path in contact being idling rollers.
To further clarify the explanation of the innovative principles of the present invention and its advantages as compared with the known art there are described below with the aid of the annexed drawing a possible embodiment as a nonlimiting example applying said principles.